Many electronic devices have the possibility of connecting to them various auxiliary devices for providing the electronic device for example with new functions. For example, a modem can be connected to the expansion slot of a computer, wherein the computer can be used for data transmission in a telecommunication network. Also, audio cards have been developed for producing different sounds with the computer. By using audio cards, it is even possible to control the computer with spoken commands. It is obvious that the audio parameters required in modem use are different from those required e.g. for playing music by means of a computer with an audio card.
For example to modern mobile stations it is possible to connect inter alia hands free units, modems, telefaxes, and computers. Each auxiliary device has its own special requirements for processing the audio signal of the mobile station in order to achieve sufficient quality of the audio signal. In a car, the acoustic environment differs from the acoustics of a room, wherein the processing of the audio signal of hands free equipment requires different reproducing qualities than when using the earphone and microphone of the mobile station itself.
This problem did not occur with earlier mobile stations, because it was usually possible to connect to the mobile station only one auxiliary device whose properties were known, and the mobile station could be optimized according to the properties of this auxiliary device.
In modern mobile stations, auxiliary devices of different types are taken into account when designing the mobile station; an attempt is made to find all the settings with which the mobile station would operate in an optimal way with all the auxiliary devices intended to be connected with the mobile station in question. However, it is thus not possible to achieve the best possible setting with each of the auxiliary devices, but the audio parameters stored in the mobile station are often a compromise between different features.
One problem with the methods of prior art is that a change in the parameters cannot be easily implemented afterwards, because the introduction of new parameters in the device would require installation of a new circuit containing inter alia the read-only memory of the digital signal processor. In practice, possible new auxiliary devices and their features will be first taken into account in devices of the next generation. Even this involves the problem that the parameters are optimized to be generally used, wherein compromises cannot be avoided. Further, one problem is that deadlines for introducing new products on the market are very strict, wherein it is usually not possible to make changes in the parameters at the last moment, but the changes are left to the next version.
Further, all users do not necessarily even need all the possible auxiliary devices that can be connected to a mobile station, wherein for such a user the optimization of the parameters is not always the best alternative.
The International Patent Application No. PCT/FI95/00005 previously filed by the Applicant introduces an arrangement for adjusting the signal level in mobile stations. The method presented in this publication is based on the fact that the most suitable signal levels obtained for different auxiliary devices are stored in the mobile station, and upon connecting the auxiliary device, the mobile station recognizes the type of the auxiliary device, wherein the mobile station retrieves from the stored information the value corresponding to the auxiliary device in question and adjusts the audio signal level accordingly. Thus the system presented in this publication applies for each auxiliary device type a certain pre-set value stored in the mobile station. Consequently, the manufacturer of the mobile station should already at the manufacturing stage know the most advantageous setting value of the audio signal level for each auxiliary device.
The best possible superimposing is not always obtained by changing the signal level only, but also other adjustment parameters are needed for influencing the travel of the audio signal in the mobile phone. In current mobile stations, the processing of audio signals takes place primarily in a digital signal processor (DSP) comprising a central processing unit (CPU), a read only memory (ROM), a random access memory (RAM), as well as means for connecting the digital signal processor with the other electronics in the device. The instruction set of the digital signal processor is designed to be adaptable especially in signal processing. The application software of the digital signal processor can implement very different types of signal processing operations particularly for audio signals, such as low pass, high pass and band pass filters, signal counting, echo and noise suppression, graphic and/or parametric equalizators, etc. according to the need. Thus the program codes required for making the desired operation are stored in the application software, for example to implement band pass filtering. In addition to the program code, the digital signal processor must still be given the parameters according to which each signal processing operation is to be carried out. For example in band pass filtering, the frequency values defining the pass band must be given, e.g. the lower limit frequency and the upper limit frequency, as is known to an expert in the field. The program code and the parameters are typically stored in the read only memory ROM.